This invention relates to the preparation of aqueous suspensions of talc and, more particularly, is concerned with a method of treating talc in order to make it more readily dispersible in aqueous media.
By the term "talc" there is meant herein a mineral comprising at least 60% by weight and preferably at least 80% by weight of true mineralogical talc, i.e. hydrous magnesium silicate having the theoretical molecular composition 3MgO. 4SiO.sub.2.H.sub.2 O. (see also "Industrial Minerals and Rocks" published by The American Institute of Mining Metallurgy and Petroleum Engineers, New York 1960, pages 835-836).
Talc has a water-repellent, or hydrophobic, crystal surface. This property makes it very difficult to wet talc with water and as a result the preparation of an aqueous suspension containing a high proportion by weight of talc is expensive in terms of time and energy. The use of talc as a paper coating pigment has hitherto been severely limited because paper coating compositions are conventionally in the form of aqueous suspensions of one or more pigments and one or more adhesives. The solids concentration of such a composition is governed by the need for the composition to be sufficiently fluid to enable it to be spread evenly over the surface of a paper web by coating machinery and yet to contain the minimum amount of water since the latter must subsequently be removed from the coated paper by thermal evaporation.
The problems caused by the hydrophobic nature of the surface of talc have been overcome hitherto by introducing a wetting agent into the water used for suspending the talc. However, the wetting agents or surfactants known at present are expensive and increase substantially the cost of using talc as a paper coating pigment. They also have a tendency to produce foam and an antifoaming agent must often be used in conjunction with them.